This invention relates to a method of producing water-insoluble alkali metal salts of carboxyalkyl cellulose.
Various methods have been known for producing a water-insoluble and water-absorptive material starting from a water-soluble alkali metal salt of carboxyalkyl cellulose by heating the water-soluble salt of carboxyalkyl cellulose in the presence of a strong acid such as hydrochloric acid, monochloroacetic acid or hydrochloric acid formed in situ upon heating monochloroacetic acid in the presence of water.
For example, Japanese Patent Publication No. 935/1979 discloses a method in which carboxyalkylation of cellulose is performed with an amount of alkali metal hydroxide and a slightly excess of carboxyalkylating agent relative to the alkali metal hydroxide, followed by heating the reaction mixture. A part of excessive carboxyalkylating agent which remained unreacted during the initial carboxyalkylation step is decomposed in the subsequent step to form hydrogen chloride which serves to catalyze the insolubilizing reaction of carboxyalkyl cellulose.
U.S. Pat. No. 3,723,413 discloses a method in which the initial carboxyalkylation reaction is performed conventhionally to form water-soluble carboxyalkyl cellulose. Then a portion of the carboxyalkylating reactants and by-products formed during the carboxyalkylation reaction is removed from the reaction mixture, and the resulting carboxyalkyl cellulose containing the remaining carboxyalkylating reactants and by-products is heat-treated at an elevated temperature for a sufficient period time for rendering the carboxyalkyl cellulose water-insoluble. This method also utilizes hydrogen chloride formed by the decomposition of monochloroacetic acid in situ for catalyzing the insolubilizing reaction.
These known methods suffer from certain disadvantages. Since the carboxyalkylating agent and hydrogen chloride formed in situ are of highly corrosive nature at an elevated temperature, the reaction must be carried out in an apparatus made of expensive alloys having high corrosion resistance. This is because the carboxyalkylation reaction includes a strong alkaline stage and thus conventional glass-lined reactors cannot be used. Even with such corrosion-resistant materials, the apparatus is susceptible to corrosion to an appreciable degree.
The method disclosed in the above cited U.S. patent calls for the step of removing a portion of the carboxyalkylating reactants and by-products formed during the initial carboxyalkylation reaction.
In order to attain a uniform quality in the final product, the amount of the reactants and by-products carried over to the next heat-treatment step should be controlled as constant as possible relative to the carboxyalkyl cellulose to be treated. Experiments have shown, however, that this control is rather difficult to achieve and the final product tends to vary in quality from batch to batch. Finally, since the treating temperature is relatively high, e.g. from 120.degree. C. to 195.degree. C., water and other volatile liquids must be droven off prior to the heat-treatment. This requires not only a complicated operation and apparatus but also a large amount of energy consumption.